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The moon, a luminous disk in the inky sky, appears suddenly above the broad crescent of Earth’s horizon. The four astronauts in the Orion crew exploration vehicle have witnessed several such spectacular moonrises since their spacecraft reached orbit some 300 kilometers above the vast expanse of our home planet. But now, with a well-timed rocket boost, the pilot is ready to accelerate their vessel toward the distant target ahead. “Translunar injection burn in 10 seconds ... ” comes the call over the headset. “Five, four, three, two, one, mark ... ignition....” White-hot flames erupt from a rocket nozzle far astern, and the entire ship—a stack of functional modules—vibrates as the crew starts the voyage to our nearest celestial neighbor, a still mysterious place that humans have not visited in nearly half a century. The year is 2020, and Americans are returning to the moon. This time, however, the goal is not just to come and go but to establish an outpost for a new generation of space ­explorers.

The Orion vehicle is a key component of the Constellation program, NASA’s ambitious, multi­billion-dollar effort to build a space transportation system that can not only bring humans to the moon and back but also resupply the Internation­al Space Station (ISS) and eventually place people on the planet Mars. Since the program was established in mid-2006, engineers and researchers at NASA, as well as at Lockheed Martin, Orion’s prime contractor, have been working to develop the rocket launchers, crew and service modules, upper stages and landing systems necessary for the U.S. to mount a robust and affordable human spaceflight effort after its current launch workhorse, the space shuttle, retires in 2010.